Fix: Use correct default in Curves to CurveEval conversion

Currently the code expects the radius attribuet to always exist on the
input Curves. This won't be true in the future though, so the correct
default value of one should be used when creating the data on CurveEval,
where the data is not optional.

source/blender/blenkernel/intern/curve_eval.cc

@@ -432,7 +432,7 @@ std::unique_ptr<CurveEval> curves_to_curve_eval(const Curves &curves)
     }
     spline->positions().fill(float3(0));
     spline->tilts().fill(0.0f);
-    spline->radii().fill(0.0f);
+    spline->radii().fill(1.0f);
     curve_eval->add_spline(std::move(spline));
   }